seed vitality after storage

In article <DHCHzM.Jxs at postoffice.ptd.net>, mal at postoffice.ptd.net (Mary
Anne Lynch) wrote:
> Now, say you plant two seeds of idential varieties - one is fresh, and
> the other has been in storage for a long time, but has just enough
> energy to germinate. Both seeds are treated identically.
>> My question is: Will the seed that was stored
> produce a plant that is as healthy as, *or* not quite as healthy as,
> the plant produced by the fresh seed? If both are quite healthy,
> is this because once they emerge from the ground, photosynthesis
> takes over, and it doesn't matter how much 'energy' was left in the
> seed, just as long as it had enough to germinate? Or, will this
> stored seed produce a plant that will forever be inferior to the fresh
> seed?
As seeds age, their ability to germinate does indeed drop. Before they
lose their ability to germinate, their vigor declines. Their ability to
germinate can be measured accurately and repeatably. In fact, the
Association of Official Seed Analysts publish the specific riteria that
are to be used to measure particular seed lots, and it is the results of
these tests that are printed on seed packets.
Higher germination numbers mean that more seeds will germinate, but it
often also means that they will have higher vigor. It would be nice to
have some vigor number printed on the packet, but that has been very
difficult to meaure accurately--it depends on too many other things also.
Aging happens at different rates depending on storage conditions, hot and
wet being harder on the seeds. The rates of decline have been determined
for many species and can be calculated for a given set of conditions from
what are known as the "Roberts equations", from a series of papers and
monographs published by Roberts and Ellis. These are useful, but there's
more to it. Some of that is in a really interesting book by David
Priestley (Seed aging : implications for seed storage and persistence in
the soil, 1986, Comstock Associates: Ithaca, NY).
Does the vigor decline so fast that it is a problem? One example is in
tropical Mexico where corn is an important crop. With the heat and
humidity, some of corn varieties will lose as much as 20% germination and
a substantial amount of vigor in the 5 months between harvest and next
season's planting. That is big trouble.
Seedlings need to get off to a good start to make a strong plant. Low
vigor seeds (with less energy and more internal damage) don't make
seedlings that get off to as good a start as fresh high-vigor seeds.
--
Thomas Bjorkman Dept. of Horticultural Sciences Cornell University